Apparatus of utilizing a travelling mold tunnel is commonly used for the production of continuous plastic tubing. The mold tunnel is formed between a pair of complementary mold assemblies each of which comprises an endless run of mold blocks which are articulately interconnected. Each mold block of one mold assembly is complementary to a respective mold block of the other mold assembly, the mold blocks cooperating in pairs along the forward run to form an axially extending mold tunnel defining a tubular mold. The wall of the mold is dependent on the inner face of the mold blocks and is the matter of choice.
The mold assemblies are driven in synchronism with one another so that the mold blocks circulate along the endless tracks. The mold blocks are always located similarly with respect to the direction of their movement. Thus when the direction of movement changes, the orientation of the mold blocks changes with it. For example, the mold blocks on the return run are totaled 90.degree. with respect to mold blocks on the tunnel run. The entrance to the tunnel may be located suitably with respect to an extrusion nozzle for extruding an annular parison of plastic against the mold surface of the tunnel. When the resulting tube is required to have more than one wall, the extrusion nozzle will extrude coaxial parisons of plastic extrudate.
Such apparatus is described in U.S. Pat. No. 3,981,663 to Lupke, issued Sep. 21, 1976 and U.S. Pat. No. 3,832,429 to Charpentier 1974. These patents describe apparatus in which the mold assemblies are arranged one above the other. Other conventional apparatus in use is described, for example in U.S. Pat. Nos. 3,677,676 to Hegler 1972, and 3,751,541 to Haglet 1973. The apparatus of these patents shows the mold assemblies arranged side-by-side. Both these conventional configurations of apparatus have particular problems associated with them, in that the independent mold assemblies must be carefully synchronized in operation so that the pairs of cooperating mold blocks come together precisely to form the mold tunnel. Moreover, to provide a pair of independent mold assemblies requires considerable duplication of machinery and equipment and, in the case of the side-by-side layout, the apparatus covers an appreciable area of floor space which may be an appreciable consideration where space is limited or expensive.
Any services which must be supplied to the mold blocks must be provided in duplicate for each mold assembly. Such services include the provision of suction lines, hydraulic fluid lines, possibly heating or cooling services, etc., maintenance and repair work is also duplicated for the mechanism of each endless track. U.S. Pat. No. 4,439,130 issued Mar. 27, 1984 to Dickhut discloses the use of mold blocks which are hinged together and closed to form the mold tunnel and opened to release formed pairs of tube. This makes possible for a single conveyor but the mold blocks still change orientation with changes in their direction of travel.
The use of hinged mold blocks which are closed to form the mold tunnel and are opened to release a formed tube is disclosed in the DE-B-1 171 596. The hinged mold blocks are fixed to a conveyor chain and circulate with this chain arround sprocket-wheels. During their travel on the return path the mold blocks change their direction over an angle of 360.degree..
Since the 1970's various improvements and modifications have been made to the basic type apparatus disclosed in the patents referred to but except for the manufacture of helically corrugated tube no other basic type of apparatus is known. For the production of helically corrugated pipe, a ribbon of molten thermoplastic has been wound on an advancing mandrel. It is not with this type of apparatus that the present invention is concerned.